1. Field of the Invention
The present invention relates to an electron gun for cathode ray tubes (CRTs), and more particularly, to an electron gun for CRTs with a helical multi-lens electrode assembly.
2. Description of the Related Art
Resolution of a CRT depends mainly on the diameter of electron beams to landing on the screen, which have been focused and accelerated by an electron lens of the electron gun. The diameter of electron beams decreases with a decrease in spherical aberration of the electron lens, and such a small spherical aberration is achieved by increasing the aperture size of electrodes of the electron gun. An electron gun having larger-aperture electrodes is desired for better resolution. However, there is a limit to increasing the aperture size because the neck, in which the electron gun is installed, correspondingly should increase, which requires greater deflection power and increased deflection distortion as a result. Also, the bead glass that supports multiple electrodes of an electron gun in place limits the aperture size.
In order to overcome this limitation, a multi-stage lens technique, in which a plurality of electron lenses are arranged for a reduction of spherical aberration, was proposed. However, this technique consumes many electrodes and results in a complicated electron gun, thereby increasing the manufacturing cost with a low product reliability. Also, there is a problem in that the length of electron gun limits the number of electrodes to be accommodated therein.
The alternative to the multi-stage lens is a helical multi-lens electrode assembly. FIG. 1 shows a cross section of a helical multi-lens electrode assembly 10. Such as helical structure of the electrode assembly 10 is obtained by coating a high-resistive layer 14 on the inner surface of a glass tube 12 in a helical pattern. Once different voltages are applied to the two ends of the electrode assembly 10, voltage drops in each pitch of the helix occurs, and electron lenses, as many as the pitches of helix, are formed due to voltage potential differences between adjacent pitches. The helical type lens can offer much smaller spherical aberration than prior large-aperture electron guns the aperture of which is recessed. Reportedly, it is not possible to implement the equivalent spherical aberration with the larger-aperture electron gun since the aperture size is beyond the aperture of the neck (refer to Society of Information Display, 1998, digest, pp. 429). Such a helical multi-lens electrode assembly is made by first forming a high-resistive layer on the inner surface of a glass tube and mechanically removing some of the coating in a helix pattern.
However, such a mechanical formation of helix is a challenging task since it requires a high precision, resulting in a low productivity. Moreover, unless the coating density and thickness, and the section of helix are maintained uniformly within one assembly, or from one assembly to another, its variation in terms of quality becomes too large to be commercially viable. A multi-lens electrode assembly made this way may accumulate charges on the uncoated glass surface and negatively affect the internal pressure in the tube because of possible presence of resistive particles after mechanical grinding away of the resistive coating in a helix pattern. Further in order to fixedly arrange such a conventional helical lens type electrode assembly in an electron gun, metal projections or claws as commonly used in the art should be attached on the electrode assembly to be embedded in the bead glass. This requires a highly complex metal-glass bonding technology.
At least for the reasons mentioned above no CRT employing helix-patterned multi-lens electrode assembly has been commercialized.
It is an objective of the present invention to provide an electron gun for cathode ray tubes (CRTs) with a helical multi-lens electrode assembly, which is easily made and yet reduces spherical aberration substantially.
To solve the above problems, it is an objective of the present invention to provided an electron gun for a cathode ray tube with a helical multi-lens electrode assembly including a helical resistive coil and two auxiliary electrodes. Each of the auxiliary electrodes has claws to be embedded in bead glasses to hold the helical resistive coil in the electron gun. When voltages are applied to the helical resistive coil through the auxiliary electrodes coupled to both ends of the coil, voltage drops occur in each pitch of the helical resistive coil, creating a plurality of electron lenses.
Preferably, each of the auxiliary electrodes has a stepped hollow cylindrical extension at one end thereof, and the auxiliary electrodes are coupled to the helical resistive coil by fitting the stepped hollow cylindrical extensions into the both ends of the helical resistive coil. Preferably, each stepped hollow cylindrical extension of the auxiliary electrodes has helical grooves, the helical grooves being fitted with the inner diameter and pitches of the helical resistive coil, and the stepped hollow cylindrical extensions are screw coupled to the both ends of the helical resistive coil. If there is a difficult in integrally forming the stepped cylindrical extension in auxiliary electrodes, the stepped hollow cylindrical extensions may be separated formed and then welded to the auxiliary electrodes.
Preferably, a projection is formed in the middle of the helical resistive coil for electrical connection to a metal lead such that a focus voltage is applied to the middle of the helical resistive coil. Alternatively, a metal lead may be formed at the middle of the helical resistive coil such that a focus voltage is applied to the middle of the helical resistive coil.